Friedewald (1972) Formula: LDL = TC - HDL - TG/2.17 (mmol/L) Reference.

"Iranian" (2008) Formula:    LDL = TC/1.19 + TG/0.81 – HDL/1.1 – 0.98 (mmol/L) Reference.

Comment:
For those of us on a low carbohydrate lifestyle - the "Iranian" formula used above can perhaps give a better "estimation" of LDL when our Triglycerides are nice and low. However the Iranian study was made with people with elevated cholesterol. Hopefully one day, a study may be done with "healthy" people (and thus develop a more accurate formula)!

TEST CASE: (to ensure calculator is working correctly on your computer).
Total Cholesterol = 5.2 (mmol/L)
HDL Cholesterol = 1.92 (mmol/L)
       Triglycerides = 0.5 (mmol/L)
LDL (estimated) = 3.0495852534562213 (mmol/L) as per Friedewald formula
LDL (estimated) = 2.2615773043224032 (mmol/L) as per "Iranian" formula
Full accuracy is shown, normally we would round-off to .1 mmol/L